The present invention relates to a cylinder liner, and more particularly, to a type thereof fitted in a cylinder wall of a cylinder block.
Conventionally, a cylinder block is formed of an aluminum alloy and a cylinder liner formed of cast iron is fitted in the cylinder block in order to reduce overall weight and to improve thermal conductivity of an automobile engine. An inner peripheral surface of the cylinder liner is subjected to honing in order to exhibit lubrication oil controllability and an initial break-in property.
For the honing process, a honing tool having a honing head 5 is employed as shown in FIG. 6. The honing head 5 has a body 6 in which a push rod 7 provided integrally with a taper cone 8 is disposed movably in an axial direction of the body 6. An elongated grindstone or a hone 10 extending in an axial direction of the body 6 is fixed to a stone holder 9 provided at an outer peripheral surface of the body 6. Several parts of the stone holder 9 radially inwardly protrude toward an axis of the body 6, and a tapered surface 9a in mating relation to the taper cone 8 is formed at each inner end of the protruding part.
If the push rod 7 is moved downwardly by a hydraulic drive means (not shown), the stone holder 9 is moved radially outwardly by the taper cone 8, so that the grindstone 10 is pressed onto a surface of a workpiece 1, i.e., an inner peripheral surface 4 of the cylinder liner. The body 6 is connected to a drive motor (not shown) for rotating motion and to a hydraulic device (not shown) for reciprocating motion. Thus, the honing head 5 is reciprocatingly movable in a direction indicated by an arrow Va, while the honing head 5 is rotated in a direction indicated by an arrow Vr. Thus, honing is imparted on the inner surface 4 of the workpiece 1.
As a result of the honing, cross-hatching grooves "A" having a predetermined cross-hatching angle and surface roughness are formed in the inner surface 4 of the workpiece 1. A portion sectioned by the cross-hatching grooves is referred to as a plateau. A desired function of the cylinder liner such as the above-mentioned oil controllability and initial break-in property is provided by the cross-hatching grooves. Thus, the cylinder liner can provide a sufficient scuffing resistance.
In order to improve scuffing resistance, a typical grindstone for honing is GC800K or a low hardness grindstone formed of a resin or a cork. The roughness attendant to the cross-hatching is relatively large such as from 0.8 to 2.0 .mu.mR.sub.3Z even in case of the plateau honing. Incidentally, R.sub.3Z implies surface roughness measurement amount. In case of a mean surface roughness R.sub.Z defined in DIN4768, a test piece is subdivided into five individual cross-sections, and a distance between the highest peak and the deepest score with respect to each cross-section is obtained to obtain a mean value. On the other hand, R.sub.3Z implies a functional surface roughness in which 2 outliers in the top and bottom ranges are not taken into consideration in each of the 5 individual sections. Incidentally GC800K is defined in JIS R6211-1980 or ISO 525. "GC" implies material of grindstone particle, i.e., silicon carbide, "800" implies particle size, i.e., extremely fine, and "K" implies binding force of a binder for binding together the grindstone particles.